try! -> ?

Automated conversion using the untry tool [1] and the following command:

```
$ find -name '*.rs' -type f | xargs untry
```

at the root of the Rust repo.

[1]: https://github.com/japaric/untry

src/libstd/process.rs

@@ -520,7 +520,7 @@ impl Child {
             }
         }
 
-        let status = try!(self.wait());
+        let status = self.wait()?;
         Ok(Output {
             status: status,
             stdout: stdout,